Gas turbines consume large quantities of air. Gas turbines are strongly dependant on the ambient air condition for their performance. The air temperature, pressure and water content impact on the gas turbine compressor's capability to compress the air and thereby will impact on the gas turbine's performance. It is known that with high ambient temperatures follow a power output loss while at cold temperatures follows a power output increase.
There are three key parameters that have a strong impact on the performance of the gas turbine. These are the mass flow, the pressure ratio and the compression efficiency. Gas turbines are constant volume machines, i.e. they operate according to fixed geometries. This means that if the air increases in density, the mass flow will also increase. The mass flow is the most dominant engine performance parameter as the mass flow is directly proportional to the engine power output.
The power loss at high ambient temperatures has drawn attention to several innovative technologies for restoring the lost power. One widely practiced method is cooling the inlet air by evaporation of water. This can be practised by scrubbing of the inlet air in a water scrubber. As the water evaporates water latent heat is exchanged for air sensible heat. The evaporation allows the air to pick up humidity till saturation is reached. For a dry and hot desert climate this may result in very significant cooling of the air. The evaporative cooling technology has two effects on the gas turbine performance, first the lower temperature increases the density and thereby the mass flow. Second, the mass of the evaporated water is added to the mass flow.
A variant to the water scrubber is the water spray technology where nozzles positioned in the inlet air duct upstream of the compressor inlet atomize water. By pumping water at high pressure the nozzles atomize the water into a mist of fine droplets (fog). The size of the droplets is typically in the range of 5 to 30 microns. These small droplets are carried with the air stream through the duct and will evaporate before entering the compressor. However, this method requires good control of air psychometrics and engine air flow. If too much water is pumped the air may not be able to pick it up as humidity and all droplets will not evaporate. The excess water will wet the duct walls where it may cause corrosion and a flooding problem. On the other hand too little water flow will not saturate the air.
A drawback with the scrubber and the sprayer method as described above is that the cooling effect is limited by how much water the air can pick up before reaching saturation. An alternative method with a higher cooling effect is the inlet air chilling technology. This technology comprise of a refrigeration unit that cools the inlet air by the heat being picked up by the refrigeration unit. This technology allows for cooling of the inlet air to lower temperatures compared to what evaporative cooling may do. However, a draw back with this method is the high costs for the unit investment which has shown to limit the applicability of this method.
Returning to the evaporative cooling technology by spraying a water mist, this has some benefits. The cooling effect is high with respect to the relative simplicity of the equipment. At sites with a hot and dry climate typically 15% power boost is achievable. Occasionally, as high as 25% power boost has been reported. However, as the temperature changes from night to day so will the water requirement also change. Good pump equipment must be designed to provide just the right amount of water. Too much water will result in “overspray” as the air can not pick up the excess water. The excess water may harm operations as it may result in corrosion damage and flooding of the air duct. The contrary situation with too little water will not saturate the air and full cooling effect will not be accomplished. Ambient conditions vary by seasonal changes as well as by the hour. This further sets stringent requirements on the pump unit. The lower pump flow limit is set by where the accomplished power boost result is of negligible advantage and the higher pump flow limit is set by the few days of the year with exceptionally dry and hot weather. For a given weather situation and engine load situation, follows a precise amount of water to reach saturation. In practise this result in a wide range for the pump capacity. A common engineering solution is to arrange multiple pumps in a cascade, i.e. several pumps are connected in parallel where pump #1 has a small capacity, pump #2 has twice the capacity of pump #1, pump #3 has twice the capacity of pump #2, and so on. Typically five pumps make up the pump skid. By running one, two or more pumps in different combinations a very large range of pump capacities is accomplished.
Yet another method for power output augmentation is related to cooling of the compressor. Cooling of the compressor results in less compression work which, in turn, results in more surplus power available on the shaft. Cooling of the compressor is preferably practised by spraying a mist of fine droplets into the compressor gas path where the evaporation and cooling takes place inside the compressor. E.g. the nozzles are installed in the space in between the compressor discs. The rapid temperature rise inside the compressor as of the compression work acts as a driving force for the evaporation process. This technology is known as “wet compression” and is practised with success. An alternative to installing nozzles inside the compressor is to install them in front of the compressor inlet. The same wet compression will take place.
Yet another method of augmenting gas turbine power output is injecting water into the gas turbine combustor. Here the water evaporates with the hot combustion gases and form steam. The steam together with the combustion gases expands through the turbine. The turbine output is increased as of the contribution by the steam mass flow. Further, the water cools the flame which in turn will allow for more fuel to be burnt while yet maintaining the same firing temperature. This latter effect provides additional power output. Besides augmenting power, another feature of the combustor injection method is that it is very efficient in reducing NOx emissions. The formation of NOx is strongly coupled to high flame temperature. With water injection into the combustor the flame is cooled and thereby is NOx formation suppressed.
Another issue of concern is the build up of fouling in the compressor. Air breathing machines like gas turbines consume large quantities of air. Air contains foreign particles in form of aerosols which enters the compressor and adheres to components in the compressor gas path. Compressor fouling results in a change in the properties of the boundary layer air stream of the gas path components. The deposits result in an increase of the component surface roughness. As air flows over the component the increase of surface roughness results in a thickening of the boundary layer air stream. The thickening of the boundary layer air stream has negative effects on the compressor aerodynamics. At the blade trailing edge the air stream forms a wake. The wake is a vortex type of turbulence with a negative impact on the air flow. The thicker the boundary layer the stronger the wake turbulence. The wake turbulence together with the thicker boundary layer has the consequence of reducing mass flow through the engine. The reduced mass flow is the most remarkable effect from compressor fouling. Further, the thick boundary layer and the strong wake turbulence result in a reduced compression pressure gain which in turn results in the engine operating at a reduced pressure ratio. Anyone skilled in the art of thermodynamic working cycles understands that a reduced pressure ratio result in a lower efficiency of the engine. The reduction in pressure gain is the second most remarkable effect from compressor fouling. Further, fouling of the compressor reduces the compressor isentropic and polytropic efficiency. Reduced compressor efficiency means that the compressor requires more power for compressing the same amount of air. As the power for driving the compressor is taken from the turbine via the shaft, there will then be less surplus power available to drive the load.
The only known way to combat fouling is to wash it away. Washing can be conducted with the engine being shut down. The engine shaft is then cranked by the use of its starter motor while wash liquid is injected into the compressor. Fouling is released by the act of the chemicals and mechanical movement during cranking. The liquid and the released fouling material are transported to the exhaust end of the engine by the air flow. This procedure is called “crank” washing or “off-line” washing. An alternative to crank washing is “on-line” washing. Here the engine is washed while in operation. “On-line” washing is also called “fired” washing as the engine is firing fuel during washing. During on-line washing liquid is injected into the compressor while the rotor is spinning at high speed. Due to high rotor speeds and short retention time for the liquid, the wash efficacy is not as good compared to the crank wash. But the method has the advantage of allowing washing during operation.
However, even though, as indicated above, there exist a large number of different techniques for augmenting the power output of gas turbine engines, such as a stationary gas turbine engine, there is no known technique that is able to provide an increased power output over a wide range of operating conditions as the same time as it offers a cost efficient and user-friendly solution.